If the allele is sex-linked rec., then the boy's genotype is X^aY.   His X^a came from his mother, who doesn't show the trait.  So his mother must be X^AX^a.  So far, so good.   The father must be X^AY, since he does not show the trait.  Then, can we get this daughter from those parents?  The daughter has at least one X^A since she does not show the trait.   That X^A could have come from either parent.  And the daughter's other X chromosome could bear either allele.    Therefore, the allele of interest here could be sex-linked recessive.   We can't find anything in the given information to rule that out.

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